This invention relates to power field effect semiconductor devices and control circuits utilizing such devices, and more particularly, to circuits and methods for controlling normally-on semiconductor devices so that they can be operated in a normally-off mode.
Prior art field controlled thyristors have been developed for power switching applications. These devices can block current flow for both polarities of applied anode voltage, and can also conduct forward current with a low forward voltage drop. These devices have also been shown to exhibit gate turnoff capability with turn-off times of less than 1 microsecond. To obtain forward blocking characteristics in this device, it is necessary to apply a negative bias to the gate. This negative bias reverse biases the gate junction and causes a depletion layer to extend under the cathode. When the depletion layers of adjacent gate regions punch through under the cathode, a potential barrier is formed between the anode and the cathode. This potential barrier prevents the injection of electrons from the cathode to the anode and, thus, allows the device to block the current flow. However, as the anode voltage increases, the potential barrier height decreases. When the anode voltage is increased beyond a certain value for each applied gate bias voltage, anode current flow will commence. The ratio of the anode voltage to the applied gate bias is defined as a blocking gain of the device. Thus, the field controlled thyristor has a normally-on characteristic and requires the application of a gate voltage to maintain it in the off state.
Field controlled thyristors can be switched rapidly from the conducting mode to the forward blocking mode by the application of a negative gate voltage while the anode current is flowing. During gate turn-off, sufficient gate current must be supplied by the gate drive circuit so as to remove the minority carrier stored charge in the n-base and to allow the gate depletion layer to extend under the cathode to pinch off the anode current flow. It has been found that turn-off times of less than 1 microsecond can be achieved when the peak gate turn-off current is comparable to the anode current.
A conventional prior art circuit used for gating field controlled thyristors (FCT) is shown in FIG. 1. In circuit 10, current supply to the load 12 by the power supply 14 can be controlled by gating the field controlled thyristor 16 using a switch 18. When switch 18 is open, FCT 16 is in its on state and the current is supplied to the load 12. When the shown schematically by 20 and 22, is used to maintain the FCT in its forward blocking mode. The turn-off speed of the FCT is controlled by the peak gate current during turn-off. This current can be controlled by the gate resistance 22. Several problems have been encountered with the use of this gating circuit. First, the devices are normally-on in the absence of a gate bias voltage, and the circuits cannot ensure fail-safe start-up and operation. Second, the devices require a substantial gate voltage in order to operate them at large forward blocking voltages. This problem has been partially overcome in the prior art by improvements in the device structure, which have allowed the development of devices with high blocking gains. In spite of improvements of blocking gain, some gate bias voltage is necessary to maintain these devices in their forward blocking mode. Third, these devices require substantial gate drive currents to switch them from the on state to the blocking state. Thus, although turn-off times of a few microseconds have been observed, gate turn-off current gains of less than 5 are typically necessary to achieve these high turn-off speeds. These drawbacks of the field controlled thyristors have been primarily responsible for its limited application to power switching applications.